When developing oil or gas fields at sea the time factor is very significant. The time taken from the drilling of the first well being able to produce, until this well and other wells in association therewith are completed and in full production, should be as short as possible. This has to do, inter alia, with the large investments being made in the form of expensive drilling and production equipment. Also this comprehensive equipment as such should be designed and built aiming at the lowest possible total costs.